California: San Joaquin Valley sinking as a result of groundwater pumping

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Parts of the San Joaquin Valley in Central California are sinking faster than ever due to excessive groundwater pumping as the state deals with a devastating drought, a Nasa report released on 19 August.

Some areas are experiencing nearly 2" of sinking per month, a trend that could damage infrastructure such as bridges, roads and aqueducts. According to geologist and Nasa Jet Propulsion Laboratory (JPL) researcher, Tom Farr, a co-author of the report, the rates can be as much as 1ft per year.

"Well it depends quite a bit on where you are, the maximum rates are up to about a foot a year and so it's pretty significant. And it's pretty much a function of the fact that the drought is causing the farmers there to pump a lot more groundwater. As they pump the ground water the land sinks. It's likely to continue until we start having more rain and we can start replenishing the ground water table."

Long-term sinking has already destroyed thousands of private and public groundwater well casings in the agriculture-dependent valley, adding that over time more sinking could permanently reduce how much water can be stored in the underground aquifer.

"One of the concerns that the Department of Water Resources has is the effect on resources, bridges, dams, roads and things like that. Right now we are seeing some bridges are no longer above the water surface, some people say a certain bridge there in Central Valley, the water will be flowing over the bridge soon. But also if affects places like the California Aqueduct which depends on gravity for the water to flow down to Los Angeles and low spots onthe aqueduct will prevent the water from flowing as well," Farr told Reuters.

Land sinking, also known as subsidence, has happened in the state for decades due to groundwater pumping during drought conditions, but Nasa's report showed that the sinking is occurring faster now. Farr said that one of the consequences of the subsidence could be flooding once there is rainfall.

"One of the problems with the subsidence is there's new low spots that we're concerned about that will flood and of course if we start getting more rainfall, say an El Nino occurs this year that'll create a lot more water flowing over the landscape and that will collect in these low spots that we previously didn't even know about," Farr said.

The California Department of Water Resources said it will launch a $10m (£6.4m) programme to bolster conservation. The department also said that programme would be funded through the $7.5bn water bond passed by state voters last November, which was the most significant state-wide investment in water supply infrastructure in decades. According to Farr, if measures are not taken soon a certain percentage of storage capacity for groundwater could be permanently lost.

"These fine grain layers that are down in this layer cake geology that have in the central valley, if they collapse or compact too far, it turns out that they will no longer be able to absorb water so there's a certain percentage that you can lose permanently of your storage capacity. It's not the whole thing but it's a significant percentage that could be lost permanently," Farr said to Reuters in an interview at the JPL facilities in Pasadena.

The data for Nasa's report, which was prepared for the department, was based on satellite imagery showing changes to the Earth's surface over time.